You can find your purchase code in your Themeforest account. First you need to go log in, then hover over your username and click ‘Downloads’ from the drop down menu. Then you can choose to download ‘License certificate & purchase code’ (as text file). Inside you should find your purchase code – 36 character code that looks something like this: ‘xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx’.
Please check the Envato Help Center or look at this article for more information.